# Explain why the speed of a wave changes when it passes from one medium into another, and state what happens to the wavelength and frequency during this transition.

> IB DP Physics Higher Level (2023 syllabus) — C.2 Wave model · Explain · 4 marks

> A plane wave travelling through air enters a denser medium such as water at normal incidence.

## Mark scheme (4 marks)

1. The speed of a wave depends on the properties of the medium through which it travels (e.g. density, elasticity/bulk modulus).
2. As the wave enters the denser medium its speed decreases (water is denser / has a higher bulk modulus effect reducing speed for sound, or greater optical density reducing speed for light).
3. The frequency remains unchanged because it is determined by the source of the wave, not the medium.
4. Since v = fλ and frequency is constant, the wavelength decreases in proportion to the decrease in speed.

## Key terms

- [frequency](https://www.gradenine.co.uk/glossary/frequency)
- [wavelength](https://www.gradenine.co.uk/glossary/wavelength)
- [medium](https://www.gradenine.co.uk/glossary/medium)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-speed-of-a-a11c9f40) · Published by Druglandscape Ltd.